Because they are trained and all that......
Yet things like this happen.
Cops are human, some good, some bad, sometimes they snap. Sometimes they go crazy. We don't know what she did to him or any of the details. (Not making excuses for his behavior here either). We simply don't know.
But that argument about "only cops should have guns" because they are trained and psychologically tested and all that folderol falls apart with incidents like this.